Recently, examination of a method of constructing a mobile communication network using IP (Internet Protocol) is becoming very active. IETF (Internet Engineering Task Force) has defined a Mobile IPv6 specification (for example, see Non-patent document 1). Network constituent components of Mobile IPv6 are a Mobile Node (MN), a Home Agent (HA), and a Correspondent Node (CN). The MN is given a unique IP address (home address) that does not change even when the MN moves. A link with the same prefix as the home address is called a home link. A prefix indicates a network portion of the IP address.
When the MN moves to a link other than the home link (the link to which the MN moved is called a foreign link), the MN acquires an IP address in the foreign link. This address is called a Care of Address (hereinafter expressed by CoA). Any application of the MN communicates using the home address. Any application of the MN can communicate without using a Care of Address.
When the MN moved to a foreign link, the MN receives a Router Advertisement that a router existing in the foreign link transmits periodically. By detecting a prefix contained in this Router Advertisement that is different from the home address, the MN detects movement from the home link to the foreign link. A Router Advertisement message is prescribed by “Neighbor Discovery” of IPv6 (see Non-patent document 2). The message is used in order that the router notifies its existence to other nodes on the same link.
When the MN detects movement, the MN performs registration to the HA. The HA holds correspondence information (Binding information) between a home address of the MN existing anywhere than in its home link and a Care of Address received at the time of registration in Binding Cache. Next, in order to acquire a packet transmitted from the CN to the home address of the MN, the HA operates as a proxy of the MN.
A procedure in which CN transmits a packet to the destination of the MN will be explained below. The CN transmits a packet to a destination of the home address of the MN. The HA acquires the packet destined to the home address of the MN. The HA searches for the Binding Cache and acquires the CoA corresponding to the home address of the MN. The HA adds an IP header destined to the corresponding CoA to the packet that was received (encapsulation), and transmits the packet. When the MN receives the above-mentioned packet destined to the CoA, the MN will remove the IP header that was added previously (de-encapsulation) to reconstruct the original packet. The MN may perform route optimization by notifying the CN of the MN's own Binding information.
Moreover, SIP (Session Initiation Protocol) (see Non-patent document 3) attracts attention as a session control protocol in an IP network. The SIP is a protocol for controlling a session of IP multimedia communication that is made into the specification by IETF. As a typical service that uses the SIP, there is VoIP (Voice over IP). The VoIP is a technology whereby voice information is transmitted and received on the IP network. In the VoIP communication using the SIP, a virtual communication path (session) is established between two pieces of the communication equipment before a communication starts. Voice data that was converted into an IP packet is forwarded on the established communication path. In VoIP communication, the SIP controls session establishment, maintenance, and session release between two pieces of communication equipment. Media information, including the attribute of voice data etc., is determined at the time of session establishment. The communication equipment notifies media information by a SDP (Session Description Protocol) contained in an SIP message. By the SDP, various information about a session including, for example, an IP address, a port number, a media type, etc. can be described.
On the other hand, in 3GPP2 (Third Generation Partnership Project 2), standardization of the third generation mobile communication (hereinafter referred to as 3G) that uses cdma2000 1× for a radio interface is progressing. A 3G network consists of a radio access network (RAN: Radio Access Network) and a core network (CN: Core Network). The RAN controls a radio transmission function, and terminates it. The RAN usually consists of a base station and base station controller. The CN performs mobility control, call control, and service control. The CN provides a circuit switching service and a packet switching service. A function of CN for providing the circuit switching service is called a circuit switched domain. A function of CN for providing the packet switching service is called a packet switched domain. A PDSN (Packet Data Serving Node) of the packet switched domain establishes a PPP connection between itself and the MN, and forwards an IP packet that is transmitted and received between itself and an external IP network. A Mobile IP technology is applied to a 3G network that the 3GPP2 specifies.
Further, for the 3GPP2, examination of implementing the 3G network based on the All IP network architecture is advancing. In an All-IP network, an IMS (IP multimedia subsystem) is connected to a packet switched domain. A CSCF (Call State Control Function) performs session control of the All-IP network. The CSCF is divided roughly into three kinds (P-CSCF, I-CSCF, S-CSCF) by its role. A P-CSCF (Proxy-CSCF) is CSCF that the MN accesses first. An I-CSCF (Interrogating-CSCF) specifies an S-CSCF when the MN existing in a visited network accesses an SIP server in the home network. An S-CSCF (Serving-CSCF) exists in the home network, and controls and manages a session state. SIP is used for a session control protocol (for example, see Non-patent document 4). A PDF (Policy Decision Function) is a functional entity for controlling QoS in the IMS. The PDF communicates with the PDSN and the P-CSCF, and provides a QoS control function (for example, see Non-patent document 5).
[Non-patent document 1]
    IETF RFC3775, Mobility Support in IPv6 , Section 4.1[Non-patent document 2]    IETF RFC2461, Neighbor Discovery for IP Version 6 (IPv6 ), Section 3[Non-patent document 3]    IETF RFC3261, SIP: Session Initiation Protocol, Section 4[Non-patent document 4]    3GPP2 X.S0013-002-0 v1.0, All-IP Core Network Multimedia Domain, Section 4.6[Non-patent document 5]    3GPP2 S.R0037-0 v3.0, IP Network Architecture Model for cdma2000 Spread Spectrum Systems, p. 22